"The first wonder is the shore without the sea"1,2
This short wonder appears to refer to a raised beach. Raised beaches are beach material deposited on the land when sea levels were higher, relatively. They often appear as beaches left high and dry away from the current shoreline. The usual causes are the massive weight of ice pushing the land down below sea level during glaciations, or higher temperatures melting land-ice like glaciers, raising the sea level. However, the picture is complicated by the response of local geology to these forces. The best raised beaches in Britain are probably those at Jura (Details and Picture). There are some great raised beaches on the Isle of Man, which might be one reason for locating the "Mona" wonders there3. However, there are at least two good raised beaches on Anglesey: one at Red Wharfe Bay at 3m above current sea level*, and one at Ynys Llanddwyn at about the same height (Photo and details). The Red Wharfe Bay beach seems like a likely candidate as it apparently sits distinct on the top of a limestone platform, though Ynys Llanddwyn is near other wonders.
You can read more on the Science raised beaches.